A target (usually made of metal) is heated under the effect of a beam of electrons in an electronic bombardment evaporator.
As shown on FIG. 1, an evaporator comprises an electron gun 10 transmitting a jet of electrons 12 accelerated under several tens of kilovolts. A d.c. power source 18 generates on an output S1 a potential difference between the cathode 20 and the anode 22 of the electron gun 10. The path 12 of the beam is curved by a magnetic field B of several tens of Gausses. The beam 12 thus directed hits the target 14 contained in the crucible 16 and thus heats it until it reaches its evaporation temperature.
In normal use conditions, the power source 18 supplies a current Io of 10A for a voltage Vo of -30 kV between the cathode 20 and anode 22, for example. The beam of electrons 12 exposing the target 14 to radiation then possesses an incident power Po of 300 kW. However, this incident power Po is not entirely consumed by the heating of the target 14. In particular, one part of the power Po is lost via the re-emission by the target 14 of the secondary electrons
These secondary electrons are of two types:
The actual secondary electrons forming a surface emission characterized by low energy (less than 50 eV),
the backscattered electrons due to elastic impacts inside the target 14; these are comparable to the electrons reflected by the target 14.
The paths 24 of these secondary electrons are themselves also curved by the magnetic field B. The energy spectrum and the spatial distribution of these secondary electrons essentially depend on three parameters: the energy of the incident beam 12, the angle of incidence and the atomic number of the target 14.